1. FIELD OF THE INVENTION
The present invention relates to a semiconductor device and more particularly to an arrangement of a semiconductor device suitable for mounting in a thin card.
2. DESCRIPTION OF THE PRIOR ART
It has been known to mount one or more integrated circuits (IC), or very large scale integrated circuits (VLSI) in a card to be used in card like semiconductor devices such as IC cards, electronic calculators or electronic watches. One of the requirements in such card like semiconductor devices is to make them thin.
An example of the conventional card like semiconductor device is shown in FIG. 1, wherein a circuit board 11 made of a hard material such as an alloy plate is mounted with a semiconductor pellet 13 which is bonded on the circuit board 11 by an organic adhesive 12 such as an epoxy adhesive and necessary wiring terminals are formed on the upper surface of the pellet 13 which connect the circuits on the board by means of bonding wires 14. After the necessary wire bonding is completed, the entire semiconductor pellet 13 and the bonding wires are sealed by a plastic resin 15. In the arrangement of the conventional card like semiconductor device shown in FIG. 1, the thickness D1 of the pellet mounting portion of the card like semiconductor device comprises the thickness d1 of the circuit board 11, a thickness d2 of the semiconductor pellet 13 and the thickness d3 of the resin sealing portion 15. Therefore, the difficulty of making the card like semiconductor device thinner is limited by these thicknesses.
One approach of solving the problem mentioned above is to decrease the thickness of the circuit board 11 keeping the desired mechanical strength by using a strong plastic sheet such as a polyimide resin sheet. In addition to the above technique, as shown in FIG. 2, through hole 16 in the circuit board 11 so that the semiconductor pellet 13 can be embedded in the through hole 16. In the arrangement shown in FIG. 2, an annular reinforcing wall 17 is formed around the through hole 16 on the side where the bonding wires 14 protrude. The plastic resin is injected in the hole 16 and the area surrounded by the wall 17 so as to seal the bonding wires 14 and the semiconductor pellet 13.
According to the arrangement shown in FIG. 2, a thickness D2 of the pellet mounting portion of the card like semiconductor device is comprises of the thickness d2 of the semiconductor pellet 13 and a height d3 of the reinforcing wall 17 or the thickness of the resin sealing portion 15, whereby the total thickness can be decreased by the thickness d1 of the circuit board 11 in FIG. 1.
Conventionally, the semiconductor pellets 13 having a 0.4 mm to 0.5 mm thickness are used in the card like semiconductor device and in order to decrease the thickness of the card like semiconductor device, the use of a semiconductor pellet 13 thinner than 0.4 mm has been proposed. However, such semiconductor pellets thinner than 0.4 mm are not commercially available, and it is proposed to grind the semiconductor pellet to decrease the thickness thereof.
Besides, as the semiconductor pellet is usually made of hard but brittle material such as silicon, mechanical resistance is low and the semiconductor pellet is easily broken by a small external force.
In the case when the semiconductor pellet 13 is mounted on the circuit board 11 made of a mechanically strong material through the metallic bonding agent 12, since the external force can be received by the circuit board, it is possible to prevent the semiconductor pellet 13 from being subjected to the external force directly, therefore, brittleness of the silicon semiconductor pellet 13 scarcely causes damage of the pellet. However, in the case when the semiconductor pellet 13 is used in card like semiconductor device without securing the bottom of the pellet as shown in FIG. 2, the semiconductor pellet 13 is easily broken by various forces such as a shrinking force of the plastic resin 15 when the plastic resin is injected and cured or by the thermal stress caused by the difference of the thermal expansion coefficient of the sealing resin material and the semiconductor pellet 13 or the external force acting directly to the semiconductor pellet 13, whereby an abnormal condition of the IC characteristic of the semiconductor pellet 13 occurs. Particularly, in the case when the semiconductor pellet 13 is made thin by grinding the bottom face thereof, a grinding flaw may be formed on the bottom face of the semiconductor pellet 13 and such grinding flaws grows to micro cracks towards the inside of the semiconductor pellet 13, thereby resulting in concentration of the stress at the cracks. Therefore, the semiconductor pellets which are ground for decreasing the thickness of the card like semiconductor devices are easier to break compared to those which are not ground. This means that the arrangement of the card like semiconductor device shown in FIG. 2 is also unavailable for the commercial use.
Moreover, the card like semiconductor device shown in FIG. 2 has a manufacturing problem that will be described below.
According to the arrangement shown in FIG. 2, since it is impossible to secure the semiconductor pellet 13 directly on the circuit board 11, it is necessary to preliminarily put the semiconductor pellet 13 on a fixing device such as a vacuum suction device in the wire bonding process. In turn, the position of the circuit board 11 must be adjusted so that the semiconductor pellet 13 is placed near the center of the through hole 16 of the circuit board 11 by means of suitable positioning equipment. Such a process prevents an automatic assembling of the semiconductor pellet 13 on the circuit board 11 from being employed thereby causing low productivity. Moreover, in various processes from the wire bonding process to the resin sealing process, since the semiconductor pellet 13 is not secured, the bonding wires tend to be broken when there are transferred, thus yield is low.